The transition from Xamarin.Forms to .NET MAUI included a move away from multiple projects in the solution (one shared project plus one project per-platform). .NET MAUI offers a single project to keep the code base more maintainable. It also provides a "Platforms" folder that can contain platform-specific code (see https://learn.microsoft.com/en-us/dotnet/maui/platform-integration/invoke-platform-code?view=net-maui-9.0).

If you are looking to implement a "test" project for the purposes of implementing unit tests or other testing code, you can manually add a new project to your solution. You can then add your main project as a reference to the "test" project and all of the code in your main project will be available to create tests with.

This works well because it keeps your file structure cleaner, allows for implementing a unit test project, and keeps your main project smaller for the purposes of publishing the application.